DC to DC converters are frequently used to convert DC voltage, provide galvanic isolation of the output from the input, and to regulate the output. DC to DC converters are also frequently used as a portion of AC to DC power supplies. For example such power'supplies are employed in Telecommunication or Cellular Power Systems to provide isolated 24 Volt or 48 Volt power to the system batteries and paralleled load. DC to DC converters are also frequently used to convert and isolate one DC voltage from another. For example, +24 Volt cellular site power is converted to -48 Volt for co-located telecommunications equipment by using DC/DC converters.
Of the many topologies that can be used for DC-DC converters, those that are buck derived are often preferred for medium (+24 V, 48 V) and low (5 V, -3 V) voltage outputs. This is due to the non-pulsating output current and ease of control as the output voltage is directly proportional to the duty cycle of the switching devices. A common buck derived converter is the forward converter discussed in "The Forward Converter in Switched-Mode Power Supplies", Philips Application Note #474, Jul. 4, 1975. This converter features good component load factors (see "Converter Component Load Factors, A Performance Limitation of Various Topologies" Bruce Carsten, PCI'88 Munich, Germany), relative simplicity, ease of control and non pulsing output current. Another buck derived converter is the half bridge exemplified in "Switched Mode Power Supplies--Highlighting a 5 V 40A Inverter Design", Motorola Application Note #AN-737, 1974. This converter is more complex with 2 primary power switches, and has the disadvantage of 2 output windings, in the power transformer. Other commonly used buck derived converters are the full bridge PWM controlled and the phase shifted bridge converters. The component load factor of the forward converter was improved with the use of an active reset switch as disclosed in "High Power SMPS Require Intrinsic Reliability", Bruce Carsten, PCI'81 Proceedings pp. 118 to 133, Munich, Germany, September 1981.
One objective in designing a DC-DC converter is to achieve low switching losses and low EMI. This can be achieved by adding capacitors across the primary power switches and by operating the circuit in such a way to bring the voltage across them to zero before turning them on. This approach was disclosed in Jitaru U.S. Pat. No. 5,126,931 in the active reset forward converter by adding a saturable reactor, or third controlled switch, in series with the output winding of the transformer and reducing the magnetizing inductance of the transformer to allow the voltage of the main switch to ring down to zero before it is turned on.
Another class of DC/DC converters use asymmetrically controlled half bridge or full bridge switches, where either switch is on except during the switching times and the output is controlled by the relative duty cycle of the switches. Examples of such a converter are described in "Soft-Switched DC/DC Converter with PWM Control", Ramesh Origanti et al, Proceedings of Intelec 93 Paris, France September '93. The first is similar to a flyback (buck/boost) converter. The second is somewhat similar to a buck derived converter but has a non-linear parabolic transfer function with maximum output at 50% duty cycle and would be difficult to control. Another such converter topology is described in "DC/DC Converter for High Input Voltage, For Switching with Peak Voltage of Vin/2, Capactive turn off Snubbing and Zero Voltage Turn on", I. Barbi et al, PESC '98 Fukuoka, Japan May '98, and is similar to a phase shifted full bridge converter. This converter topology suffers from substantial extra current in the transformer windings when both half bridge sides are switched to the same input voltage. This circuit will have a poor transformer load factor except at 50% (full) duty cycle.
It is accordingly an object of the invention to provide a new and improved DC-DC converter most suitable for medium output voltages such as 24 Volts or 48 Volts DC.
An additional object of the invention is to provide a DC-DC converter with an isolated output which is proportional to the control duty cycle, and has a non-pulsating output current. A further object of the invention is to provide zero voltage switching of the controlled power switches (e.g. MOSFETS) and zero current switching of the output non-controlled switch (e.g. a diode) to maximize conversion efficiency and minimize EMI, without using an additional switch in series with the output. Another object of the invention is to provide a DC-DC converter with high component load factor for the switching devices and especially the transformer to minimize size and cost.